1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of surface related multiple elimination in marine seismic surveys.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. A well-known technique of geophysical prospecting is a seismic survey.
The resulting seismic data obtained in performing a seismic survey is processed to yield information relating to the geologic structure and properties of the subterranean formations in the area being surveyed. The processed seismic data is processed for display and analysis of potential hydrocarbon content of these subterranean formations. The goal of seismic data processing is to extract from the seismic data as much information as possible regarding the subterranean formations in order to adequately image the geologic subsurface. In order to identify locations in the Earth's subsurface where there is a probability for finding petroleum accumulations, large sums of money are expended in gathering, processing, and interpreting seismic data. The process of constructing the reflector surfaces defining the subterranean earth layers of interest from the recorded seismic data provides an image of the earth in depth or time. The image of the structure of the Earth's subsurface is produced in order to enable an interpreter to select locations with the greatest probability of having petroleum accumulations.
In a marine seismic survey, seismic energy sources are used to generate a seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically sound wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflected seismic energy is detected by seismic sensors (also called seismic receivers) and recorded.
The appropriate seismic sources for generating the seismic signal in marine seismic surveys typically include a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The seismic source generating the wavefield is typically an air gun or a spatially-distributed array of air guns.
The appropriate types of seismic sensors typically include particle velocity sensors (known in the art as geophones) and water pressure sensors (known in the art as hydrophones) mounted within a towed seismic streamer (also know as a seismic cable). Seismic sensors may be deployed by themselves, but are more commonly deployed in sensor arrays within the streamer.
After the reflected wave reaches the seismic sensors, the wave continues to propagate to the water/air interface at the water surface, from which the wave is reflected downwardly, and is again detected by the sensors. The reflected wave continues to propagate and can be reflected upwardly again, by the water bottom or other subterranean formation interfaces. Reflected waves that reflect more than once are termed “multiples” and are typically treated as noise. A particular category of noise comprises multiples that reflect at least once from the water surface and are called surface-related multiples.
Three-dimensional Surface-Related Multiple Elimination (3D SRME) strives to attenuate the surface-related multiples typically by a predict-and-subtract process. The surface-related multiples are first estimated from the seismic data and then the predicted multiples are subtracted from the seismic data to leave a noise-attenuated signal. A first step in this process comprises constructing a multiple contribution gather for a source-receiver trace, which involves the computation of the convolution of pairs of traces over a spatial area called the aperture. A second step comprises constructing a predicted multiple trace which contains primarily multiple reflections, which involves stacking all the multiple contribution traces in the multiple contribution gather for the source-receiver trace. A third step comprises subtracting many such predicted multiple traces from the original seismic data.
The choice of aperture for a multiple contribution gather that is easiest to implement is a rectangular spatial area with preselected inline and crossline dimensions, which is centered on the midpoint location of the source-receiver trace for which the multiple trace will be predicted. An aperture is sufficient for a particular multiple contribution gather when at least the apices of all contributing events in the multiple contribution gather fall within the aperture. This means that the surface reflection points corresponding to the apices lie within the aperture and that Fresnel stacking works to collapse the contributing events into the corresponding multiple events at the time of the apices. Hence, a simple choice of aperture for multiple contribution gathers will inevitably be a trade-off between cost and accuracy. For some multiple contribution gathers, the aperture will be too large and for others it will be too small. An optimally sized and shaped aperture would be both as small as possible to minimize computational efforts and large enough to capture all apices of all surface reflection points and for constructive interference to work in Fresnel stacking for those apices.
Thus, a need exists for a method for efficiently determining appropriate shapes and sizes of apertures for multiple contribution gathers. Preferably, the optimal shape and optimal size of the aperture can be determined dynamically from the seismic data itself.